Nozzle segments forming the stationary components for the hot annular gas path of a gas turbine typically have inner and outer walls between which are defined a plurality of circumferentially spaced vanes. Various methods are employed to cool the segments, including their inner and outer wall surfaces, as well as the surfaces of the vanes extending therebetween. For example, open-circuit air-cooling, or closed-circuit steam-cooling, or a combination of open-circuit air and closed-circuit steam-cooling may be used. In U.S. application Ser. No. 08/414,697, filed Mar. 31, 1995, entitled "Turbine Stator Vane Segments having Combined Air and Steam-Cooling Circuits," (Attorney Docket No. 839-354), there is disclosed an advanced gas turbine design having a plurality of nozzle segments disposed in annular arrays to form the stationary hot gas path components of respective first and second stages of a gas turbine. The complexities of the various multiple piping arrangements for supplying the cooling medium are such that it has been found desirable to manufacture the stationary components from a multiplicity of nozzle segments joined circumferentially to one another to form the annular array of stator vanes.
To improve overall performance and efficiency of the turbine, it has also been found desirable to provide a coating on the surfaces of the nozzle segments exposed to and defining the hot gas path. Such coating typically includes a base coat of an alloy comprising the elements nickel, chromium, aluminum and yttrium and which is applied to the base metal of the nozzle segments to provide resistance to oxidation and corrosion. On top of this base coating, there is applied a second or top coating of yttrium-stabilized zirconia having thermal insulating and anti-corrosion properties. These coatings are typically applied by a plasma spray head. However, only portions of the nozzle segments can be fully coated to the desired coating thicknesses and oftentimes portions of the nozzles are only coated with one or both of the coatings or remain uncoated. The non-uniformity of the applied coatings may result in substantial and undesirable temperature variations along the metal of the nozzles. The inability to properly coat the nozzle segments and to coat them to uniform thickness or depth along their hot gas path surfaces is a result of the physical size of the spray head and the complex configuration of the multi-vane nozzle segments. That is, the nozzle segments can be fabricated, e.g., integrally cast, in multi-vane segments having inner and outer walls with two, three or more vanes extending between the inner and outer walls, the segments thereby constituting doublets or triplets, as applicable, depending on the number of vanes in each segment. Because of the close spacing of the various portions of those nozzle segments, it has been found to be practically impossible to use a plasma spray head to apply coatings to all surfaces of the multi-vane segments or to uniformly coat these surfaces. In contrast, a single vane nozzle segment, i.e., a singlet wherein only a single vane extends between inner and outer walls, can be properly coated with the plasma spray head, ensuring a uniform coating on all hot gas path surfaces.